The present invention relates generally to manufacture and repair of machine parts, and, more specifically, to surface finishing of such parts.
Machines are assemblies of various parts which are individually manufactured and assembled. Machines typically include metal parts, although synthetic and composite parts may also be used. And, each part requires specialized manufacturing.
For example, metal parts may be fabricated from metal stock in the form of sheets, plates, bars, and rods. Metal parts may also be formed by casting or forging. Such parts may be machined to shape in various manners.
Machining requires the selective removal of material to configure the part to its final shape and size within suitable manufacturing tolerances, typically expressed in mils, and with a suitable surface finish which is typically smooth or polished without blemish.
Each step in the manufacturing process of a given machine adds time and expense which should be minimized for producing a competitively priced product. It is desirable for each subsequent step in the manufacturing process to avoid damaging previously finished portions of the part which would then require additional corrective finishing steps.
Gas turbine engines are an example of a complex machine having many parts requiring precise manufacturing tolerances and fine surface finishes. A typical engine includes a multistage compressor for pressurizing air which is mixed with fuel in a combustor and ignited for generating hot combustion gases which flow downstream through one or more turbine stages that extract energy therefrom. A high pressure turbine powers the compressor, and a low pressure turbine provides output power, such as powering a fan disposed upstream from the compressor in an aircraft engine application.
The engine thusly includes various stationary components, and various rotating components which are typically formed of high strength, state of the art metal and composite materials. The various parts undergo several steps in their manufacturing and are relatively expensive to produce.
Since the combustor of the engine must contain hot combustion gases during operation, it is formed of high strength superalloy material for maintaining strength at high temperature. An annular combustor includes radially outer and inner combustion liners which are joined together at upstream ends thereof to an annular dome. The dome includes a plurality of circumferentially spaced apart carburetors that inject fuel and air into the combustor, which is then ignited for generating the hot combustion gases contained in the combustor.
The combustor is cooled during operation by channeling a portion of compressor air through many film cooling holes extending through the liners in dense patterns for producing protective films of cooling air along the inboard surfaces thereof exposed to the hot combustion gases.
During the manufacturing process, the individual combustion liners are formed in rings which are assembled and joined together with the annular dome. The film cooling holes in the liners may be formed or drilled therethrough in various manners. It is common to laser drill small film cooling holes at an acute angle through the respective liners.
However, laser drilling results in expulsion of molten metal from the holes as they are drilled. Some of the laser expulsion becomes welded to the surface of the liner in the form of small bumps thereatop. And, some of the laser expulsion becomes welded to the inlet and outlet perimeters of the holes where they meet the outer and inner surfaces of the liner.
Prior to laser drilling, the liner surfaces are in finished form, which is highly smooth. The laser expulsion welded to the liner surfaces and to the inlets or outlets of the film cooling holes is undesirable since it adversely affects performance thereof.
Accordingly, an additional process is required for removing the laser expulsion from the liner surfaces and the holes. And, the perimeter corners of the holes where they meet the liner surfaces are preferably radiused for removing the sharp edges thereof and reducing stress concentrations thereat.
Grit blasting is a process in which abrasive particles are discharged in an air stream for abrading a metal surface. This process is quite abrasive and requires close control. A related process uses both air and water as the carrier stream for the abrasive grit and has been used to remove laser expulsion and radius the film cooling holes for a combustor liner.
However, this process requires a significant expenditure of time for removing the laser expulsion over thousands of small film cooling holes in a typical combustion liner, and requires expensive equipment therefor. The cost of the liner must correspondingly increase. And, this process has limited capability for radiusing the film cooling holes without adversely affecting surface finish of the liner, or wall thickness thereof.
Since the grit stream necessarily covers many small film cooling holes and liner surface therebetween, care must be used to minimize degradation of the finish of the liner surface between the holes during the removal of laser expulsion.
Accordingly, it is desired to provide an improved process for surface treating a workpiece, having little or no adverse effect on surface finish thereof.